In many parts of the world, valuable metals such as gold, nickel and platinum group metals (PGM's) occur in iron-bearing sulphides such as pentlandite, pyrrhotite and arsenopyrite. These minerals are recovered selectively from the ores by flotation. While flotation is a remarkably efficient process, one of its most significant limitations for iron-bearing sulphides is that fine particles are not recovered efficiently and a great deal of fine valuable sulphides are lost to the tailings. For example, for pentlandite which is a nickel-iron sulphide it is not unusual for as much as half the nickel which fails to float in a nickel concentrator to be less than 10 .mu.m in size. The improvement of fine particle recovery has been the subject of a great deal of research, much of which has focussed on the use of different types of flotation cells such as column cells.